Use $releasever over RPM macro in repository configuration on RHEL

[SteenSchutt]

Description

While it's not really a big deal, RHEL has the $releasever variable with contains the major release of the operating system. Using this instead of inserting the version at file creation time makes it slightly more portable when migrating between OS versions, and it also aligns with the use of $basearch in the very same line :)

Links

Fixes my minor nitpick

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